Vecna

Vecna was born over two millennia ago on the Prime Material Plane of Oerth. His mortal history is shrouded in secrecy, and the first time he comes into the history books, he is an unusually powerful lich. He raises a massive army and lays waste to much of Oerth, conquering it under his banner. For several hundred years, he ruled his empire, always seeking expansion. His chief lieutenants were the vampire Kas and the demilich Acerark.   One fateful day, a group of clerics of the god Ptolus strike him with a mighty ray of light, searing away the left side of his body and face. But even this is unable to stop him, as he comes back to full health and enacts a bloody and terrible revenge. During this time, it is believed he wrote the Book of Vile Darkness, his personal spellbook.
When it seems like Oerth is totally lost, Kas betrays Vecna and strikes him down with the blade Vecna had forged for him. Kas’s reasons for his betrayal are unknown, and Vecna’s empire soon crumbles without him.
Vecna next appears several hundred years later, as a lesser deity. While only a theory, Diesa and Myr had proposed that he spent the time in between in Harmonia, gathering the materials needed for the ritual that eventually made him a god.
He created four artifacts that contained pieces of his divinity - the Hand of Vecna, the Eye of Vecna, the Book of Vile Darkness and the Sword of Kas the Betrayer. He then began to put plans into motion.
First, he needed a realm to call his own. He determined that he liked the Doomguard’s Citadel of Ash, Cavitus, and he stole it from them, treating it as his stronghold and moving it to an unknown location. From there, he began to experiment and study.
He designed a ritual that would allow him to absorb the powers of a demigod to ascend to greater godhood. He then tricked the demigod Iuz into coming to Cavitus, where he was bound by a ritual and absorbed by Vecna, causing his ascension.
Vecna’s enemies at this point are too numerous to name, as he put forth many schemes to attain greater power, not all of which worked. Realizing he may have overplayed his hand, Vecna goes into hiding in Ravenloft, where he strikes a bargain with the Dark Masters who rule that demiplane.
But going into hiding was not the only reason he went to Ravenloft. Vecna knew the demiplane Ravenloft would come unusually close to Sigil soon, and he theorized he could essentially “jump off” of Ravenloft and enter Sigil physically, bypassing the Lady of Pain’s protections against deities.
Having succeeded in that plan, Vecna and the Lady of Pain engaged in a terrible duel. They seemed evenly matched, but Vecna was overwhelmed by supporters of the Lady of Pain, who broke his concentration and allowed the Lady to slay him. When Vecna was slain, Iuz reappeared, and the Lady quickly sent him to Oerth.
For most people, Vecna’s story ends there, roughly seventy-five years ago.